The present invention relates to objective arrangements particularly, but not exclusively, for use in overhead and liquid crystal display (LCD) projection systems.
Overhead projectors and LCD projectors are well known. In an overhead projector arrangement as shown schematically in FIG. 1, a slide 100 is laid on a horizontal light bed beneath an optical objective 102 having a substantially vertical axis. A mirror 104 above the objective reflects the image out onto a reflective screen 106, for viewing.
In many situations, it is necessary to tilt the mirror to project the image higher up the screen, in order for it to be visible by the whole audience. When this is done, the distance over which the image is projected to the top of the screen is longer than the distance over which the image is projected to the bottom of the screen. Hence, the width of the image is greater at the top of the screen than at the bottom and consequently, an ostensibly rectangular image will appear on the screen with the distorted shape 108 shown in FIG. 2. This type of distortion is known as the keystone effect, by analogy with the shape of an architectural keystone.
FIG. 3 shows one possible arrangement for dealing with the keystone effect. This arrangement seeks to keep the optical magnification constant across the field. It can be seen from FIG. 3 that the image plane 110 is not perpendicular to the optical axis 112, but is parallel to the object plane 114, with the result that the keystone effect will be corrected, because the ratio L (top) to L (top)xe2x80x2 will be equal to the ratio of L (bot) to L (bot)xe2x80x2 (where L (top) and L (bot) are object distances at respective points in the field, and L (top)xe2x80x2 and L (bot)xe2x80x2 are image distances for the relevant field). However, the image plane and object plane will no longer be optically conjugate when a tilted objective is used. Other aberrations of the objective will therefore be very large and this method cannot be used when a large effective focal length is required, or high resolution is required.
Similar problems arise in relation to LCD projectors. However, the LCD array plane is preferably arranged to be tele-centric, i.e. not tilted relative to the objective, in order for the LCD array to have more energy output. However, the arrangement in FIG. 4 has been proposed, and interposes a Fresnel lens 120 between the LCD plane 122 and the objective 124, to produce a tilted intermediate virtual image of the LCD plane. The tilt introduced by the Fresnel lens 120 gives the virtual image a keystone shape but the intermediate virtual image can be conjugate with the plane of the screen 126, to reduce other aberrations, and the keystone effect introduced by the Fresnel lens is opposite to that introduced by a tilted screen, so that the overall degree of keystone effect can reduce.
The present invention seeks to provide improved projection arrangements, particularly, but not exclusively for use with LCD arrays.
The invention provides an objective arrangement for use in a projection system which provides an image plane tilted relative to the optical axis, the arrangement being rotationally asymmetric.
The projection system may have a field lens, preferably a Fresnel lens. Preferably the objective arrangement comprises a plurality of elements, preferably three elements, and preferably at least one element is off-set from the principal optical axis of the arrangement, to provide rotational asymmetry. Preferably the off-set element is the final element of the arrangement.
The arrangement may be a fixed focus or vari-focus arrangement, and is preferably a zoom arrangement. Preferably the or each fixed elements is off-set and the or each movable element is aligned with the principal optical axis.
The degree of asymmetry is preferably selected substantially to balance rotationally asymmetric aberrations caused by the tilt of the Fresnel lens. The degree of asymmetry may be selected substantially to reduce or correct astigmatism caused by the tilt of the Fresnel lens.
The objective arrangement may be constructed according to the measurements set out in Appendix B below.
The invention also provides a projection system comprising a planar image source, a Fresnel lens tilted relative to the image plane, and an objective arrangement substantially as set out above, the system being arranged to cause an image from the image source to be projected through the Fresnel lens and objective arrangement.